Various X-ray transmissive monitoring electrodes have heretofore been made to facilitate X-ray examination of a patient without requiring removal of the electrodes or significantly impairing the X-ray image. Examples of X-ray transmissive monitoring electrodes are disclosed in U.S. Pat. Nos. 4,050,453; 4,257,424; 4,370,984; 4,674,511; 4,685,467; 4,442,315; 4,539,995; and 5,265,579. Monitoring electrodes are very small, for example in the order of a few square centimeters and need only carry very low electrical signals in the order of milliamps. In general, such X-ray transmissive monitoring electrodes are not capable of conducting and distributing the high levels of energy required in transcutaneous stimulation and defibrillation electrodes.
Defibrillation electrodes must be capable of conducting the high energy level required for defibrillation, up to 360 Joules or more and must also distribute the energy over a relatively large area of the patient's epidermis to achieve adequate current density distribution within the ventricles. The ANSI/AAMI presently specify that the minimum active area of individual self-adhesive electrodes used for adult defibrillation and pacing shall be at least 50 square centimeters and that the total area of the two electrodes shall be at least 150 square centimeters.
Some prior defibrillation electrodes such as disclosed in U.S. Pat. Nos. 4,895,169 and 5,330,526, provide an electrically conductive metal plate having an area to substantially cover the skin contacting gel pad, for distributing the energy over the area of the pad. U.S. Pat. No. 4,748,983 provides an electrode body formed of a porous granular or fibrous carbon impregnated with an electrolytic solution that covers the upper surface of a skin contacting gel pad. Some other stimulation and defibrillation electrodes disclosed in U.S. Pat. Nos. 4,226,247; 4,239,046; 4,722,354; 5,356,428 and 5,366,497 provide a conductive backing for covering the skin contacting pad and which is formed of expanded metal or metal strands that are woven or formed into an open mesh.
Some of the above patents indicate that the backing is radiolucent or have at least some degree of X-ray transmissivity. In addition, some of the above patents also indicate that the defibrillation electrode disclosed in the patents are combination electrodes that can also function as monitoring electrodes between defibrillation pulses. However, it remains a problem to improve the X-ray transparency of defibrillation electrodes and the ability of the defibrillation electrodes to reliably function as monitoring electrodes promptly after transmission of a defibrillation pulse through the electrode. In addition, problems have been encountered with prior art stimulation and defibrillator electrodes, particularly after application of repeated high level defibrillation or cardiac pacing pulses, with irritation and burning of the patient's skin due to high current density around the perimeter of electrodes.